The Oryza officinalis complex is the largest in the genus Oryza, containing diploid species with BB, CC, or EE genomes and tetraploid species with BBCC or CCDD genomes. Several species in this complex such as O. officinalis, O. australiensis, and O. minuta have been intensively used in rice breeding programs because of their useful genes for disease and insect resistance (Brar and Khush 1997). Three closely related tetraploid species, O. latifolia, O. alta, and O. grandiglumis, from Central and South America were reported as allotetraploid with the CCDD genomes, even though no diploid species has been found as the D genome donor in Oryza.
Some researchers believe that the DD genome originated from the AA genome, whereas others think it differentiated from the CC genome. The first proposition should be ruled out because the CD genome species has a greater genetic distance from the A-genome species than species containing other genomes (Wang et al 1992, Aggarwal et al 1999). Recent studies indicated that the DD genome and EE genome (from O. australiensis) are most closely related, and that the ancestor of the EE genome might have played an important role in the formation of the CD-genome species (Wang et al 1992, Ge et al 1999). The relationships between the DD and CC or DD and EE genomes, however, still need clarification.